This invention relates generally to carburetors and more particularly to diaphragm type carburetors.
Carburetors are currently used to supply liquid hydrocarbon volatile fuels to a wide range of two-cycle and four-cycle internal combustion engines including hand held engines such as engines for chainsaws and weed trimmers as well as a wide range of marine engine applications. Diaphragm type carburetors are particularly useful for hand held and other engine applications wherein the engine may be operated in substantially any orientation including up side down. Typically, diaphragm carburetors have been used with two-cycle engines and there is a continuing struggle to reduce the exhaust emissions of these engines to prevent escape to the atmosphere of hazardous hydrocarbon vapors and to comply with increasingly strict governmental regulations regarding the same. Still further, engine manufacturers are continually seeking carburetors which are easy to calibrate and of low cost to manufacture and assemble.
A typical diaphragm type carburetor has a diaphragm fuel pump which draws liquid fuel from a fuel tank and delivers that fuel under pressure to a fuel metering system which provides a metered quantity of fuel into a stream of air flowing through the carburetor for delivery to the engine. Undesirably, a significant amount of fuel vapor may exist within the fuel system, such as within the fuel tank and/or fuel lines leading to the carburetor. Fuel may also tend to vaporize within the carburetor to provide still further fuel vapor within the fuel system. This fuel vapor tends to collect in various portions of cavities and passages within the carburetor to form large vapor bubbles and large clusters of vapor bubbles. When the large vapor bubbles, clusters or portions thereof are drawn along with the liquid fuel supply and delivered to the engine, the fuel supply to the engine can become inconsistent or undesirably lean resulting in unstable engine operation, especially for engines equipped with fuel injection apparatus. To combat this, the carburetor may be set to provide a somewhat richer than required fuel and air mixture to ensure adequate fuel is supplied to the engine. However, this may increase the hydrocarbon content of the engine emissions. These problems are exasperated with engine operation at ambient temperatures above 80xc2x0 F. which causes the rate of fuel vapor generation to increase significantly.
A carburetor having a plurality of restricted fluid flow paths upstream of a fuel and air mixing passage to inhibit the passage of large fuel vapor bubbles through the fuel paths and to the fuel and air mixing passage. Desirably, the restricted flow paths constrain large volume vapor bubbles and clusters from passing therethrough undistributed to prevent an inconsistent or overly lean fuel and air mixture from being delivered to the engine. At least two restrictive flow paths diffuse, separate, disperse and/or break up the large vapor bubbles and clusters into a plurality of smaller vapor bubbles which are more uniformly separated and distributed within the liquid fuel flowing through the carburetor to provide a more consistent flow of fuel to the engine.
In one form, the restrictive flow paths comprise a plurality of small openings in the carburetor body which define an outlet of a fuel metering chamber from which the fuel is supplied to the fuel and air mixing passage. In another form, the restrictive flow paths are defined through a porous material disposed between a fuel pump and the fuel metering chamber of the carburetor downstream of the fuel pump. Both the porous material between the fuel pump and fuel metering chamber and the plurality of holes defining the outlet of the fuel metering chamber may be used in the same carburetor, if desired.
Objects, features and advantages of this invention include providing a carburetor which provides a more stable supply of fuel to an engine, reduces the hydrocarbon exhaust emissions from the engine, facilitates calibrating the carburetor for a particular engine or engine family, prevents large volumes of undispersed fuel vapor from being delivered to the engine all at once, improves engine operation especially at ambient temperatures above 80xc2x0 F., provides a pressure restriction between the fuel pump and fuel metering chamber to improve pressure regulation and fuel flow through the carburetor, is of relatively simple design and economical manufacture and assembly, reliable, durable, and in service has a long useful life.